KR102000560B1 - Midsole having a double layer and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a midsole having a double layer and a method of manufacturing the same, wherein the above-mentioned object is achieved by providing a high-hardness primary composition constituting a surface layer of a double layer and a secondary composition of a soft material having a lower hardness Preparing a composition; Injecting the primary composition into a mold; And injecting the second composition into the first composition, wherein the first composition and the second composition differ in flowability from each other. As a result, a different kind of composition having different viscosity and crosslinking reactivity can be used to prepare a midsole having different physical properties or different colors on the surface and inside in one mold.

Description

[0001] The present invention relates to a midsole having a double layer and a manufacturing method thereof,

The present invention relates to a midsole having a double layer and a method of manufacturing the same. More particularly, the present invention relates to a midsole having a double layer, and more particularly, The present invention relates to a midsole having a double layer for producing the midsole and a method of manufacturing the same.

In the construction of a typical shoe, it is possible to form individual parts using materials of different properties by various factors such as purpose, suitability and aesthetics in shoes and similar fields, or to embody another process technology to realize properties of two or more materials Research is being conducted. Especially, among the components of shoes, the midsole of the shoe has been much studied depending on the nature of the raw material.

For example, in the case of a polar rubber or a resin material having a polar group or a halogen atom, the surface of the polar rubber or the resin material is easily activated according to molecular properties, which is advantageous in that the outsole or upper part of the shoe has good adhesion and slip resistance . In addition, due to the polar nature, molecular motion is reduced by increasing intermolecular attraction such as hydrogen bond and van der waals force, so that properties such as permanent compression set, hardness, Has a high value. However, due to low molecular mobility, not only the wearability of users such as cushioning and shock absorption but also high processing temperature, tear, tear strength, elongation, specific gravity ) Have low characteristics. On the contrary, non-polar rubber and resin materials are activated according to their molecular properties, and thus have excellent tensile, tear strength, elongation and specific gravity as well as wearability of users such as cushion and shock absorption, Properties such as hardness have low characteristics.

Since the rubber and resin material exhibits various properties depending on the physical properties of the rubber and resin materials, the midsole or the outsole is manufactured using the characteristics. On the other hand, when different requirements are required for the shoe, for example, the shoe heel requires a large impact absorbing function and the forward part requires a rebound resilience function.

The major problem in the field of midsole for shoes is that, when different physical properties are required for the upper, middle and lower parts of the midsole, it is necessary to manufacture parts according to their respective properties according to these requirements and to assemble them through the assembly process . For example, the midsole may be formed through injection molding of the midsole components other than the upper, middle, and lower parts of the midsole, respectively, through bonding, reheating, and other processes. In addition to the environmental problems caused by the organic solvent and the energy loss in the reheating process, both of the bonding processes cause problems such as various molds such as the upper mold, the middle mold and the lower mold, and the manufacturing process time increase and the manufacturing cost increase. Also, due to rapid changes in physical properties, it causes not only limitations of ponit of failure and control of physical properties, but also cause injury to the wearer's lower extremity due to stress.

Korea Patent Office Registration No. 10-1231548 Korea Patent Office Registration No. 10-1402361

Accordingly, an object of the present invention is to provide a midsole having a double layer in which a surface and an interior of a mold are made to have different hardness or different colors of the midsole using different compositions of different viscosities and cross-linking reactivities, Method.

The above object can be achieved by a method for preparing a secondary composition comprising a primary composition having a high hardness constituting a surface layer of a bilayer and a secondary composition of a soft material having a lower hardness than the primary composition, Injecting the primary composition into a mold; And injecting the second composition into the first composition, wherein the first composition and the second composition are different in flowability from each other.

Here, it is preferable that the primary composition has a lower melting index and higher viscosity than the secondary composition, and has higher polarity and hardness than the secondary composition, and each of the primary composition and the secondary composition contains chloroprene Rubber, chloroprene rubber (CR), ethylene vinyl acetate (EVA), urethane, thermoplastic polyurethane (TPU), natural rubber (NR), isoprene rubber A peroxide crosslinking agent, an antioxidant, a foaming agent, and a pigment in combination with a pigment selected from the group consisting of acrylic resin, acrylic resin, acrylic resin, acrylonitrile-butadiene rubber, butadiene rubber, polypropylene, polyethylene and mixtures thereof.

In addition, the step of injecting the primary composition into a mold comprises injecting the primary composition at 90 to 150 캜 into the mold at 150 to 190 캜 at an injection pressure of 45 to 80 bar and an injection speed of 9 to 15 rpm, The step of injecting the secondary composition into the interior of the primary composition may comprise injecting the secondary composition at a temperature of 80 to 110 占 폚 lower than the injection temperature of the primary composition into the interior of the primary composition at a rate of 110 to 175 bar And an injection speed of 65 to 95 rpm.

The above object is also achieved by a surface layer comprising a first composition of a high hardness of a double layer; And an inner layer made of a second composition of soft material having a hardness lower than that of the first composition, wherein the first composition and the second composition are different in flowability from each other .

According to the above-described constitution of the present invention, a different kind of composition having different viscosity and crosslinking reactivity can be used to prepare a midsole having different physical properties or different hues on the surface and inside in one mold.

1 is a flowchart of a method for manufacturing a dual layered midsole according to an embodiment of the present invention,
Figures 2 and 3 are cross-sectional photographs of a midsole having a bilayer of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a midsole having a double layer according to the present invention and a method of manufacturing the same will be described in detail with reference to the drawings.

First, the midsole having the double layer according to the present invention is characterized in that the surface layer is made of a hard material having a relatively high hardness and the inner layer is made of a soft material having a lower hardness so that the function of the midsole can be fulfilled do. That is, in the case of the midsole, it is preferable that the rear portion is relatively thicker than the front portion and the shock absorbing function is performed, and the front portion is thin and performs the rebound resilience function. And the surface layer touching the sole should be protected from breakage such as tearing.

Therefore, in order to perform such a function, it is desirable that the rear portion of the midsole be soft in comparison with the front portion, and the surface layer should have a high-hardness property with a dense structure to prevent tearing or the like. Conventionally, in order to perform such a function, the respective parts having separate functions have been molded and then adhered or adhered to each other. However, the above-described problems arise. The inventors of the present invention have solved this problem fundamentally by preparing one midsole having different physical properties from the inner layer and the surface layer in one mold by using a heterogeneous material having different physical properties after a long period of research.

According to the present invention, there is a focus on the production of a midsole constituting a double layer of an inner layer and a surface layer in one mold. For this purpose, a surface layer composed of a first composition and a second composition disposed inside the surface layer, And an inner layer made of a secondary composition. Hereinafter, the first and second compositions according to the present invention will be described. Here, the flowability generally includes changes in viscosity due to crosslinking, material inherent viscosity, melt index, pressure, and rotation speed.

The first composition means a composition disposed on the surface of a double-layered midsole to form a surface layer, and the secondary composition means a composition disposed inside the midsole to form an inner layer. That is, a secondary composition is disposed inside the primary composition to form a midsole having a double layer.

These first and second compositions are different from each other in flowability and have different viscosities and reaction degrees of crosslinking, but basically have polar structures such as halogen atoms, vinyl groups, amide groups, And chloroprene rubber (CR) having hydrophilicity, ethylene vinyl acetate (EVA), urethane, thermoplastic polyurethane (TPU), and mixtures thereof. (NR), isoprene rubber (IR), butadiene rubber, polypropylene, polyethylene and the like having non-polar groups or hydrophobic groups such as methyl groups, And mixtures thereof can be suitably used in combination. That is, the basic constituents constituting the first composition and the second composition may be composed of the same components, but they are adjusted so that the overall flow properties are different from each other by varying the mixing ratios of the physical properties represented by the components.

In the case of the first composition, it is necessary to prevent breakage such as tearing and to have high hardness as a surface layer of the midsole, and the secondary composition has a role of forming a midsole inner layer and particularly performing a shock absorbing function of the rear portion of the midsole It should be made of relatively soft material. Since such a material exists in the inside, it does not directly touch the outer surface, breakage such as tearing is prevented, and the function of the midsole can be faithful. The viscosity and the reactivity of these secondary compositions are from 44 to 55.

Generally speaking, a composition having a low melt index and a high viscosity corresponds to a case where the polarity is high. However, in the case of a composition having a high polarity, the composition tends to have a strong attractive force between the compositions. The midsole composed of the double layer is basically made of a material having a high hardness to be disposed on the surface layer, so that wear is reduced. In addition, the inner layer of the midsole imparts elasticity to the midsole through a composition having a low hardness, and thereby, a midsole having an excellent gripping feeling of the user can be obtained. Thus, by having lower flowability than the secondary composition, the primary composition having a higher hardness is applied to the outside of the midsole.

In addition, since the first composition has a high viscosity and has a stronger property to stick to the surface of the mold than the second composition, when the second composition is injected into the mold, the first composition is pushed into the mold and the first composition sticks to the surface of the mold, . Also, when such a composition having a high hardness is applied to the first composition, even if the second composition is injected, the primary composition is not disturbed and is pushed into the mold in a state of being united with each other, .

Compared to such a primary composition, the secondary composition has a low viscosity and a low degree of crosslinking. The second composition has a high melt index and low hardness, and exhibits a high impact absorption and low specific gravity tendency as compared with the first composition because it is a non-polar composition.

The flow properties of the primary and secondary compositions are most influenced by injection pressure, injection rate and melt index of the composition. The melt index of the composition depends on the composition of the parts by weight of the composition. For example, 19 parts by weight of vinyl acetate (VA), 25 parts by weight of ethylene vinyl acetate (EVA) having a melt index of 2.5 and a content of a polypropylene-polyethylene copolymer (PP-PE copolymer) having a melt index of 1.2 , 75 parts by weight of a primary composition having a VA content of 22 parts by weight, an EVA with a melt index of 2 of 75 parts by weight, and a PP-PE copolymer having a melt index of 1.2 of 25 parts by weight, Has a lower melt index and a higher viscosity than the second composition and thus has low flowability.

In more detail, a composition having low flowability generally has a high polarity, a high degree of crosslinking and a low melt index. In the case of a composition having a high polarity and a high degree of crosslinking, the composition tends to have a strong attractive force, . The midsole composed of the double layer is basically composed of a material having a low hardness and a low degree of wear due to its high hardness, and the inner layer having a low hardness and high elasticity. This makes it possible to obtain a midsole having an excellent gripping feeling of the user. Therefore, by having lower flowability than the secondary composition, the primary composition having a higher hardness is applied to the surface layer.

In order for the primary composition to have higher hardness and lower flowability than the secondary composition, a method of increasing the addition amount of a polar group such as a vinyl group, an amide group and the like and a material having a high crosslinking reaction Can be used. For example, when a composition is prepared with a composition based on ethylene vinyl acetate (EVA), if the content of vinyl acetate (VA) is increased according to the kind of EVA, a first composition having an increased polarity can be obtained, When the content of vinyl acetate is relatively reduced, a non-polar secondary composition can be obtained. As the VA content of EVA increases, crystallinity decreases and physical properties become weaker.

The peroxide crosslinking agent, the antioxidant, the foaming agent, the pigment and the like are dry-blended, dispersed through the Soviet Union and the kneading process together with the basic constituents of the first and second compositions having such physical properties, Thereby forming a composition and a secondary composition, respectively.

The surface layer of the midsole can be variously adjusted depending on the shape of the mold and the position of the gate, and each has advantages and disadvantages. First, in a standard mold for typical Middlesold molding, the thickness of the surface layer can be configured to be as thin as 1.5 mm or less. When applied to a standard mold, economical efficiency for mold production is reduced, but the process complexity due to the extruder and mold movement to achieve two or more colors, the productivity drop of 10 to 15% and the graying of the gate portion easily occur. Secondly, in a two color mold for forming a product having two colors at the time of forming the midsole, the thickness of the surface layer is preferably in the range of 1.5 to 5 mm for the midsole. In the case of such a mold, a middle plate which is conventionally used for a two-color mold is not used.

Next, the process of fabricating the midsole using the first composition and the second composition will be described.

First, the prepared primary composition is injected into a mold having a midsole shape. Here, the mold is not particularly limited among the molds generally used for manufacturing the midsole, and it can be applied without difficulty to existing molds. At this time, it is preferable that the primary composition is injected by 20 to 35% by weight when the amount injectable into the entire mold is 100% by weight as described above. If the amount of the primary composition is less than 20% by weight, the surface layer becomes thinner, causing problems such as warping and hardness drop. When the amount exceeds 35% by weight, the surface layer becomes excessively thick, The defects of the midsole may occur.

When the primary composition is injected, the temperature of the injection mold is preferably 150 to 190 ° C. When the temperature of the injection mold is lower than 150 ° C, the primary composition can be cured while being injected into the mold. The properties of the primary composition may change. At this time, the temperature of the primary composition to be injected is preferably maintained at 90 to 150 ° C.

The injection pressure of the primary composition is 45 to 80 bar, and the injection speed is preferably 9 to 15 rpm. If the injection pressure of the first composition is 45 bar and the injection speed is less than 9 rpm, the first composition may not be easily injected into the mold and may be clogged at the inlet. If the injection pressure is 80 bar and the injection speed exceeds 15 rpm, 20 to 35% by weight is required to be filled from the inlet, but it is not filled from the inlet, and there is a fear that it is injected at the inner depth by high pressure and high speed. As a result, it is mixed with the secondary composition internally to cause problems in the physical property dispersibility.

The secondary composition is then injected into the interior of the primary composition.

A secondary composition is injected into the interior of the primary composition injected into the interior of the mold to form a midsole. At this time, it can be seen that the secondary composition has physical properties different from those of the primary composition, that is, a high melt index, a low viscosity, and a reactive crosslinking degree, as described above.

The injection temperature of the secondary composition is from 80 to 110 캜, which is slightly lower than the injection temperature of the primary composition, and the temperature of the mold is maintained at 150 to 190 캜. In order to crosslink and cure the first composition and the second composition after injection, the crosslinking holding time is preferably 200 to 400 seconds. If the injection temperature is less than 80 ° C, the second composition may not be partially melted and may be injected in an uneven composition state. If the injection temperature exceeds 110 ° C, a portion of the first composition may melt, The shape load can not be maintained due to the problem of pressure load and low viscosity according to the present invention.

The pressure for injecting the secondary composition into the primary composition is from 110 to 175 bar and the injection speed is from 65 to 95 rpm at a higher pressure than the pressure at which the primary composition is injected and at a higher rate. The injection pressure and rate of the primary composition correspond to very low pressures and rates as the primary pressures for injecting the primary composition into the mold. Therefore, if the second composition is injected at a lower pressure and speed than the first composition, the first composition can not be pushed into the mold, and the first composition and the second composition are mixed, making it impossible to produce a proper double layer midsole. Particularly, when the injection pressure is less than 110 bar and the injection speed is less than 65 rpm, the secondary composition can not properly push the primary composition to the innermost side of the mold, so that the primary composition is disposed on the outside and the secondary composition is formed on the inside Can not. In addition, when the pressure exceeds 170 bar and 95 rpm, the secondary composition may be sprayed with excessive pressure to form a layer with the primary composition, and the composition may be mixed by pressure.

The work should be done at 5 seconds (sec) or less after the injection of the primary composition upon injection of the secondary composition. This is because when the injection of the second composition is delayed, the first composition slowly crosslinks and cures and the second composition is not injected properly. That is, the work is carried out within a range in which the hardening and the degree of crosslinking of the primary composition maintain the melting property by the temperature.

Even when pressure is applied to the secondary composition during injection, the pressure decreases as it enters the mold. Therefore, the prepared dual-layered midsole contains a large amount of the secondary composition in the front portion and a larger amount of the primary composition in the rear portion than the secondary composition. Therefore, when the user wears the shoes including the midsole, the user can feel the cushion feeling at the front portion and the back portion is hard, which improves the wearing comfort.

Hereinafter, embodiments of the present invention will be described in more detail.

The following Table 1 shows the composition ratios of the first composition and Table 2 shows the composition ratios of the second composition. In the case of the first composition, the surface layer is preferably composed of the inner material in the case of the second composition.

Here, EVA is ethylene vinyl acetate, VA is the content of vinyl acetate contained in ethylene vinyl acetate, and PP-PE co-polymer is a copolymer of polypropylene (PP) and polyethylene (PE). Zn / St stands for zinc stearic acid and St / A stands for stearic acid.

Primary composition ingredient Weight portion MI Main material EVA (VA 19%) 15 2.5 EVA (VA 28%) 10 2.5 PP-PE co-polymer 75 1.2 Cross-linking agent ZnO 3 - BIPF [F] One - Crosslinking auxiliary Zn / St One - St / A One - TAC 0.2 - blowing agent JTR-D One -

Secondary composition ingredient Weight portion MI Main material EVA (VA 22%) 60 2 EVA (VA 19%) 15 2.5 PP-PE co-polymer 25 1.2 Cross-linking agent ZnO 3 - BIPF [F] 0.95 - Crosslinking auxiliary Zn / St 3 - St / A One - blowing agent JTR-D One -

In this case, the values of Shore A 79 and PP-PE Copolymer Shore A 87 for EVA (VA 19%) Shore A 95, EVA (VA 22%) Shore A 87 and EVA (VA 28% denotes, in the case of density EVA (VA 19%) 0.940g / cm 3, EVA (VA 22%) 0.945g / cm 3, the case of EVA (28% VA) 0.950g / cm ^3, PE-PP Copolymer 0.885g / cm < ³ > That is, the weight portion is determined based on the melt index, hardness, and density. The first composition has high physical properties through a composition based on PP-PE having a low melt index and a high hardness, and the second composition has a high melt index and low physical properties to realize desired properties. At this time, the melt index of the first composition has a value equal to or smaller than the melt index of the second composition. When the melt index of the first composition has a higher value, the operation of mixing at the above-mentioned pressure and injection speed causes mixing of the first and second compositions.

Comparison test of the weight ratio of the first composition and the second composition Primary composition (%) 5 10 15 20 25 30 35 40 45 50 Secondary composition (%) 95 90 85 80 75 70 65 60 55 50 A Part
Surface layer (mm) 0 0.1 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 A Part
Inner layer (mm) 8 7.9 7.8 7.75 7.70 7.65 7.60 7.55 7.50 7.45 B Part
Surface layer (mm) 0.15 0.2 0.25 0.5 0.75 One 1.25 1.5 1.75 2 B Part
Inner layer (mm) 19.85 19.8 19.75 19.50 19.25 19.00 18.75 18.50 18.25 18.00

In Table 3, the comparison was made between the first composition as the surface layer and the second composition as the inner layer, and the average size of adult male and female was 260 mm. At this time, the thicknesses of the surface layer and the inner layer according to the weight were compared according to the weight of the front part of the midsole, A part 8 mm, and the rear part B part 20 mm. In the working conditions, the mold temperature was set to 168 ° C and the crosslinking time was set to 360 seconds.

According to the above-described embodiment, when the primary composition is 20 to 35% and the secondary composition is 65 to 80%, it is suitable to form the surface layer of 0.25 to 0.4 mm for the A part and 0.5 to 1.25 mm It is possible to obtain a suitable physical property as a midsole.

Inner layer of midsole Pressure according to example size (mm)
Inner layer Injection. Pressure (bar) Zone.1 Zone.2 Zone.3 Zone.4 Zone.5 230 114 114 114 112 112 240 118 118 118 116 116 250 122 122 122 120 120 260 126 126 126 124 124 270 130 130 130 128 128 280 134 134 134 128 128

Midsole inner layer Speed according to the embodiment size (mm) Inner layer Injection. Pressure (rpm) Zone.1 Zone.2 Zone.3 Zone.4 Zone.5 230 70 70 70 68 68 240 74 74 74 72 72 250 78 78 78 76 76 260 82 82 82 80 80 270 86 86 86 84 84 280 90 90 90 88 88

Based on the above-mentioned embodiment, it is preferable that the secondary composition of the primary composition and the secondary composition have a melt index of 130 to 135% higher than that of the primary composition through mixing of the resin. Also, when the weight ratio is 164.5 g based on Table 3 based on Table 3 and the total weight is 164.5 g, the surface layer constitutes 21.5% and the inner layer constitutes 78.5%. Under the above conditions, the conditions of Tables 4 and 5 indicate the speed and pressure for discharging the amount of 1.3 to 1.5 g / sec for the first composition and 85 to 90 g / sec for the second composition on the basis of 250 mm . In the case of extruding the composition, it is preferable to set the conditions of the extruder according to each section under the condition of 90 to 150 DEG C for the above composition, and 150 to 190 DEG C for the injection mold.

The multi-layered midsole according to an embodiment of the present invention prepared through the first and second compositions has a surface layer formed through the first composition having a hardness (C type) of 55 to 60, a specific gravity of 0.2 to 0.25 g / cc, a tensile strength of 25 to 30 kg / Cm < 2 >, an elongation of 450 to 520%, a breaking tear of 3.7 to 4.5 N / mm, a permanent compression ratio of 50 to 54%, an elasticity of 45%, a tearing of 0.19 to 0.21 N / mm and an adhesion of 191 to 207 kgf / The inner layer formed through the composition has a hardness of 46 to 49, a specific gravity of 0.19 to 0.21 g / cc, a tensile of 27 to 31 kg / cm2, an elongation of 470 to 520%, a tear of 3.7 to 3.9 N / mm, 60% of elasticity, and 3.0 to 3.4 N / mm of tearing tear.

The multi-layered midsole manufactured through such an embodiment can be seen from FIGS. 2 and 3. FIG. 2 is a cross-sectional view of a multi-layered midsole formed by injecting a first composition through a mold and injecting a second composition into the first composition. Respectively. It can be seen that the first composition of the multi-layered midsoles determined through cutting is composed of black color and the second composition is white color, and these two colors are clearly distinguished without being mixed with each other. Also, it can be seen that the secondary composition decreases in the section A to C, which means that as the secondary composition enters the inside, the injection pressure decreases and the amount of injection decreases. Such a midsole can feel a cushion feeling on the front side and a hard side on the back side, so that a midsole having a state suitable for a user can be obtained. This can also be confirmed from FIG. 3, and FIG. 3 likewise shows that the amount of the secondary composition decreases from section A to section E.

Conventionally, in order to obtain a multi-layered midsole, a plurality of separate molds are prepared, respectively, and a midsole foam is prepared by bonding the above-mentioned midsole foam through an adhesive. However, in the case of the present invention, the multi-layered midsole can be obtained by a simple process without using a separate adhesive in one mold by using the melting index (MI) and the difference in viscosity of the first and second compositions . In addition, conventionally, since separate layers must be prepared, it is difficult to make the thickness of the multilayer to a very thin level. However, in the case of the present invention, if the content of the first composition is reduced, a very thin multilayered midsole .

Claims (5)

A method for producing a midsole having a bilayer structure,
Wherein the bilayer structure comprises a core-skin layer formed on the skin so that the second composition is positioned in the core and the first composition completely surrounds the second composition, Surface layer structure,
Preparing the secondary composition of the high hardness surface layer of the double layer and the secondary composition of a soft material having a hardness lower than that of the primary composition to form an inner layer;
Injecting the primary composition into a mold;
Injecting the second composition into the mold before the first composition is completely cured without opening the mold, and injecting the second composition into the first composition,
The primary composition and the secondary composition differ in flowability from each other,
In the step of injecting the secondary composition into the interior of the primary composition, the secondary composition is injected from the front side of the mold corresponding to the front side of the midsole, so that the primary composition of the secondary composition And the thickness ratio of the second layer to the second layer is reduced. The method according to claim 1,
Wherein the first composition has a lower melt index and higher viscosity than the second composition, and has a higher polarity and hardness than the second composition. The method according to claim 1,
Wherein each of the primary composition and the secondary composition comprises,
Chloroprene rubber (CR), ethylene vinyl acetate (EVA), urethane, thermoplastic polyurethane (TPU), natural rubber (NR), isoprene rubber A peroxide crosslinking agent, an antioxidant, a foaming agent, and a pigment, which are formed by mixing a resin selected from the group consisting of ethylene, propylene, butadiene rubber, IR, butadiene rubber, polypropylene, polyethylene and mixtures thereof, ≪ / RTI > The method according to claim 1,
The step of injecting the primary composition into a mold comprises:
Said primary composition at 90-150 DEG C is injected into said mold at 150-190 DEG C at an injection pressure of 45-80 bar and an injection speed of 9-15 rpm,
The step of injecting the secondary composition into the primary composition comprises:
Wherein said secondary composition at a temperature of 80 to 110 DEG C lower than the injection temperature of said primary composition is injected into said primary composition at an injection pressure of 110 to 175 bar and an injection speed of 65 to 95 rpm higher than said primary composition A method for manufacturing a midsole having a double layer. As a midsole having a bilayer structure,
Wherein the bilayer structure comprises a core-skin layer formed on the skin so that the second composition is positioned in the core and the first composition completely surrounds the second composition, Surface layer structure,
A two-layer superficial layer made of the primary composition having a high hardness;
The second composition being different in flowability from the first composition and being soft at a hardness lower than that of the first composition, and after the first composition is injected into the mold, the first composition is completely cured Wherein the secondary composition is formed by injection into the interior of the primary composition inside the mold,
Said inner layer comprising:
Wherein the secondary composition is injection molded from the front side of the mold corresponding to the front side of the midsole to the inside of the primary composition so that the thickness ratio with respect to the surface layer decreases from the front side toward the rear side.

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